Weapon-grade (or "bomb-grade") uranium or plutonium is any alloy or oxide compound that contains enough of certain isotopes of these elements to serve as the active ingredient in a nuclear weapon. Some civilian weapon-grade materials are tracked by international organizations, especially the United Nations' International Atomic Energy Agency (IAEA) and the European Atomic Energy Community (EURATOM), to prevent diversion to bombs. The goal is to prevent nuclear proliferation, that is, the possession of nuclear weapons by unauthorized nations and/or groups.
Those states that already had nuclear weapons at the time of the treaty's creation—the U.S., United Kingdom, France, Russia, and China—are not subject to IAEA safeguards. Only four states—Cuba, India, Israel, and Pakistan—have not signed the NPT and are not part of any international safeguard system. Of these four, India and Pakistan have nuclear weapons and Israel is widely assumed to have a nuclear weapons.
The IAEA tracks weapon-grade materials (or, in the case of plutonium, dilute materials that could be refined to weapons grade) in non-military nuclear fuel cycles in states that are signatories to the Non-Proliferation Treaty (NPT) of 1968.
EURATOM safeguards civil plutonium and uranium in the European countries, including materials not covered by mandatory IAEA safeguards under the NPT (i.e., those in the UK and France). The IAEA and EURATOM cooperatively safeguard European materials to avoid redundancy.
Military nuclear materials are tracked only by the governments that own them. Because the tracking techniques employed internally by nuclear- weapons states vary from nation to nation and are always partly or wholly secret. (more)